The present invention relates to equipment to be used in connection with an extrusion apparatus as well as corresponding methods for the extrusion of material; in a particular aspect it relates to a process chamber provided in front of a die thereby serving to control parameters of the extrusion process.
As a method for extruding a billet, i.e. a rod of a metal material to be extruded, direct extrusion has conventionally been prevailing due to the lower cost of tooling, however, in some cases the indirect extrusion method is adopted due to its merit of producing comparatively less friction loss in the course of an extrusion process.
The indirect extrusion method referred to herein means a method for extruding a billet through a tool in the form of a die without relative movement between the container and the billet. More specifically, indirect extrusion is a method in which an axially movable container, while accommodating a billet, is moved towards a die arranged in front of a stationary hollow die stem arranged concentrically with the container, a first end of the billet being forced towards the die by an extrusion or press ram.
Normally, on the circumferential surface of the billet there are impurities, which should be prevented from being extruded through the die as this would result in a finished product comprising these impurities. The outer portion of the billet comprising the impurities is known as the shell portion.
In order to solve this problem U.S. Pat. No. 4,459,837 proposes a die comprising a billet facing end portion having a slightly smaller diameter than the inner diameter of the through-going opening in the container, the resulting opening between the die and the inner surface of the container leading to a circumferential recess arranged on the peripheral surface of the die, the recess thus providing a circumferential space between the die and the container. In the course of extrusion, the shell portion is accommodated in the annular space between the die and the container wall. More specifically, in the disclosed embodiment the recess is provided with a plurality of axially arranged separating walls, thereby creating a plurality of recesses, for the purpose of dividing the flowed-in shell of the billet into pieces so that they may be removed easily.
After the extrusion of a billet, i.e. after each stroke of the container and press ram towards the stationary die, it is necessary to remove the shell impurities, which have accumulated in the recess. According to U.S. Pat. No. 4,459,837 this is done by using a free die which after the extrusion of a billet is removed from the end of the hollow stem. After removal the die is cleaned in order to remove the impurities, which have been collected in the recess, which is normally done by etching. As it is an object to run as many extrusion cycles per time unit as possible, it is necessary to provide a plurality of dies such that a cleaned die can be mounted in front of the hollow stem immediately after the container and press ram have been withdrawn in order to run a new extrusion cycle. Typically six dies are required.
As appears from the above, the known method requires that a plurality of dies specifically adapted for indirect extrusion is provided by the inclusion of a peripheral recess on each die. Further, for each extrusion stroke the die has to be removed, and subsequently cleaned, and a new, cleaned die mounted in front of the hollow stem.
Accordingly, it is an object of the present invention to provide improved extrusion methods and equipment therefore which meet one or more of the following requirements: Higher efficiency for running repeated extrusions, lower costs for tooling and dies, and lower associated costs.
It is another object of this invention to provide extrusion methods and equipment therefore wherein the life of the die and other parts is elongated by reducing the forces acting on the different structures.
Although the invention primarily addresses indirect extrusion, it will be apparent from the following that many aspects of the invention have corresponding relevance for direct extrusion.
Firstly, higher efficiency is achieved by a method and apparatus allowing the die to be effectively cleaned in a time-effective manner without having to remove it, this allowing for shorter periods between each effective extrusion stroke. In a further aspect, higher efficiency is provided by avoiding the need for removing the so-called residual, i.e. the rest of the billet positioned between the press ram and the die and the end of an extrusion stroke. Further again, cost-reduction is provided by an arrangement in which a single standard die can be used, i.e. the type of die normally used for direct extrusion and not being provided with a recess for collecting the shell portion of the billet.
More specifically, according to a first aspect of the present invention, a process chamber is provided in front of the die serving to control parameters of the extrusion process. In a preferred embodiment a die holder is provided which serves both as a means for mounting a die on the front end of a hollow stem as well as provides a process chamber in front of the die during extrusion. The die holder comprises recess means at its leading end, i.e. the end facing the billet and comprising an inlet opening, this allowing a standard die to be used. The recess means may be in the form of the above-described circumferential recess or a plurality of recesses separated from each other. Although the term diameter traditionally is used to describe such a circumferential structure, in the context of the present application, diameter is also used to describe the relative dimensions of non-circular structures. These considerations also apply to the term bore.
In a further preferred embodiment the process chamber is provided with means for controlling the flow through the chambers inlet opening, for example in the form of resistance means arranged in the inlet opening. More specifically, such a control means can be adapted to serve both as a temperature regulating means for achieving an optimum temperature of the metal inside the process chamber, as well as controlling the position of the metal inside the process chamber during loading of a new billet, i.e. controlling the above-described residual. To control the position of the residual is an important aspect, as a melted residual would otherwise tend to flow out of the chamber. Depending on whether the chamber is to be used for direct or indirect extrusion, the resistance means may be used with or without the shell-collecting recess.
Controlling the temperature as described above has a number of advantages. During extrusion most of the heat necessary for providing a given desirable process temperature for the metal to be extruded through the opening in the die is produced as a consequence of shear forces as the metal is deformed and made to flow in given directions. Normally, the largest contribution to heat generating takes place as the metal is forced through the opening in the die or around bridge inserts arranged close to the die opening, however, this results in substantial wear on the die as well as the large forces necessary tend to deform the die. On the contrary, by providing heat build-up corresponding to the inlet portion of a process chamber, wear and the forces acting on the die can be considerably reduced. Further, by optimising the configuration of the heat generating means extrusion can take place at a lower pressure, which again results in less wear on the die as well as reduces the requirements as to the extrusion press as such.
In the context of the present application, the die, the chamber and other associated structures may be described as a die assembly, however, this term also covers embodiments in which the different components are formed integrally with each other.
According to a second aspect of the present invention, an improved dummy block is provided. As described above, the billet is forced towards the die by a press ram; however, normally an additional dummy block is interposed between the foremost end of the press ram and the trailing end of the billet. Due to the high forces exerted by the press ram the dummy block has a tendency to become attached to the billet, this resulting in a problem when the residual between the die and the press ram has to be removed after each extrusion stroke. According to U.S. Pat. No. 4,459,837 a free dummy block is used which is removed together with the residual (after this has been separated from the die) for subsequent cleaning. Another traditional method of freeing the dummy block from the residual is to apply some kind of adhesion preventing means, such as graphite, grease or soot, before a new billet is loaded. Although this method leaves impurities on the residual, this is normally not a problem as the residual, together with these impurities, is separated from the die.
In contrast to the above, the dummy block of the present invention allows for both easy separation from the residual as well as leaves a residual with a xe2x80x9ccleanxe2x80x9d trailing end surface. Apart from the direct cost-savings from not having to clean or grease the dummy block, the dummy block of the invention also allows the residual to be xe2x80x9creusedxe2x80x9d in the subsequent extrusion stroke, i.e. in contrast to the traditional method of removing the residual, the leading end of a new billet is arranged in direct contact with the free end of the residual. It is evident, that considerable cost savings will be possible in this way. In a preferred embodiment, the dummy block is used in combination with a die holder and process chamber as described above, this allowing for control of the position of the metal inside the process chamber during loading of a new billet.